Patent Literature (PTL) 1 discloses a technique of determining a viewer's eye fatigue state based on his or her gaze point position, adjustment position, and gaze coordinate value of three-dimensional video.
PTL 2 discloses a wearable camera that measures an electrooculogram.
PTL 3 discloses a technique in which, with regard to a data change in impedance caused by electrode displacement, potential measurement in response to sound stimulation with the use of electrodes attached to the head or the neck is suspended at regular time intervals to measure an impedance and, in the case where the impedance is inappropriate, the frequency of impedance measurement is increased.